Three combined pretreatments for reactive gasification feedstock from wet coffee grounds waste
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Green Processing and Synthesis 2021; 10: 169–177
Research Article
Isao Hasegawa*, Tatsuya Tsujiuchi, and Kazuhiro Mae
Three combined pretreatments for reactive
gasification feedstock from wet coffee grounds
waste
https://doi.org/10.1515/gps-2021-0016
received September 07, 2020; accepted January 24, 2021
1 Introduction
Abstract: In this study, a new pretreatment for using wet Excess wet biomass, especially food waste, is a serious
food biomass waste as a high calorific and reactive feed- environmental problem all over the world. Food waste
stock for gasification is presented. The method involves contains carbohydrates, protein, fat, and fiber and is gen-
the addition of calcium hydroxide, hot water treatment, erally used as a fertilizer or feed for domestic animals.
and dewatering in vegetable oil. Hot water treatment at However, there are alternative supplies of fertilizer
230°C reduced the oxygen/carbon atomic ratio of coffee and feed, which are inexpensive and easier to handle.
grounds waste to improve the calorific value, but this Although food waste is continuously discharged from
treatment also formed an inactive cross-linked structure food processing plants and the foodservice industry, it
caused by dehydration reactions. By mixing the coffee is difficult to preserve because of the high water and
grounds waste with calcium hydroxide powder before protein content. In addition, food waste is expensive
the hot water treatment, cross-linking was suppressed and energy-intensive to gather or transport. Therefore,
and the gasification rate of the char significantly increased drying food waste is an attractive method to reduce the
because of the catalytic effect. With or without hot water
weight and avoid decay, but the conventional methods
treatment, the time required to complete gasification for
are less than ideal. Air-drying food waste can generate an
the chars of the grounds mixed with calcium hydroxide
unpleasant smell and can cause spontaneous ignition
was reduced to about one-third of that for the char of the
because of the heat produced by oxidation or fermenta-
untreated grounds. After heating in vegetable oil at 150°C,
tion. Superheated steam drying results in less oxidation
moisture was completely removed from the coffee grounds
of the food waste [1,2] but requires an enormous amount
and they became impregnated with a large amount of the
of energy to generate the steam. In this study, we present
oil. As dewatering in oil did not affect the gasification rate
an oil dewatering method where the moisture is replaced
of the chars, a combination of these three treatments was
by oil. A dewatering method for Australian brown coal
found to efficiently convert wet food biomass waste into a
using solvents was reported by Miura et al. [3]. While
gasification feedstock.
Miura’s method was carried out under high pressure,
Keywords: wet biomass waste, gasification, calcium the dewatering presented in this study was carried out
hydroxide, hydrothermal treatment, dewatering in oil at atmospheric pressure. The vegetable oil absorbed by
the food waste can be used directly as a high-calorie
gasification feedstock. As heat is transferred between
the food and the oil in a slurry, the energy efficiency of
the oil dewatering method is high.
Conventional thermochemical conversion processes,
including combustion, gasification, and flash pyrolysis,
* Corresponding author: Isao Hasegawa, Department of Chemical, are unsuitable for wet biomass because of the heat of
Energy and Environmental Engineering, Kansai University, 3-3-35 evaporation of moisture. To overcome this problem, gasi-
Yamate-cho, Suita-shi, Osaka 564-8680, Japan,
fication in supercritical water has been proposed and
e-mail: hase7@kansai-u.ac.jp
Tatsuya Tsujiuchi, Kazuhiro Mae: Department of Chemical
actively investigated [4–8]. However, supercritical water
Engineering, Kyoto University, Katsura Campus, Nishikyo-ku, treatment under severe conditions requires a high capital
Kyoto 615-8510, Japan cost and leads to difficult operability. Furthermore, if
Open Access. © 2021 Isao Hasegawa et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0
International License.170 Isao Hasegawa et al.
complete gasification under the supercritical conditions rapeseed oil (Nakalai Tesque Inc.) was used. The analyses of
is not achieved with the addition of supplemental water, these samples are listed in Table 1.
the ammonia and nitrogen-containing organic effluent
from the food waste is also discharged [9,10]. Therefore,
the two objectives of this study were to perform the hot
water treatment and oil dewatering under more moderate 2.2 Addition of calcium compounds
conditions and to carry out the conventional gasification
at the lowest possible temperature. If wet biomass is con- A saturated aqueous solution of calcium hydroxide
verted into a high calorific and reactive feedstock that (Wako Pure Chemical Industries, Ltd.) or calcium acetate
exhibits a catalytic effect during gasification, it can be (Wako Pure Chemical Industries, Ltd.) was used as the
used as an energy resource or raw material for syngas. catalyst precursor. The coffee grounds were soaked in the
From this viewpoint, the wet biomass was treated without Ca solution using the impregnation method. These Ca-
supplemental water by adding a calcium compound as a impregnated coffee grounds were used to compare their
catalyst precursor and the hot water treatment used the gasification rates. A mixture of 2.73 g of wet coffee
moisture, which the food waste had retained. Calcium grounds directly added to 0.28 g of calcium hydroxide
compounds are known to act as a gasification catalyst powder was also prepared.
[11], and Leppalahti et al. [12] reported that limestone
reduces hydrogen cyanide in the gasification gas. Cal-
cium oxide can assist in capturing CO2 during chemical
looping gasification [13]. In chemical looping gasifica- 2.3 Hot water treatment and dewatering
tion, solid or liquid metal oxides [14] are used as oxygen in oil
carriers. As the hot water treatment promotes the dehy-
dration reaction of the hydroxyl groups in cellulose [15], The hot water treatment of the coffee grounds was per-
the oxygen/carbon atomic ratio of biomass would be formed in a small stainless steel reactor (10 mL in
reduced through this treatment, resulting in an improve- volume) that was filled with 4 g of the wet grounds and
ment of the calorific value. Furthermore, it has been either an extra 4 mL of distilled water or no extra water.
reported that a hydrothermal or oil treatment can lead No extra water was added in the hot water treatment to
to the suppression of self-ignition for coal [16,17]. In the wet coffee grounds mixed with calcium hydroxide
this article, the validity of these three methods was exam- powder. After purging with nitrogen gas, the reactor
ined (Ca-loading, hot water treatment, and oil dewa- was immersed in a sand bath and heated to a temperature
tering) as pretreatments for gasification feedstock. between 25°C and 230°C, where the reaction pressure
rapidly increased up to the saturated vapor pressure.
After 30 min, the reactor was dipped into a sufficient
amount of water to immediately cool the vessel and ter-
2 Materials and methods minate the reaction. The water-soluble products were
recovered as filtrates and analyzed using a total organic
carbon (TOC) analyzer and a liquid chromatograph (LC).
2.1 Materials The calcium-loaded coffee grounds were not filtered after
the hot water treatment to avoid leaching of calcium. The
Wet coffee grounds were used as raw materials. The
coffee grounds contained 63.4 wt% of water when received
Table 1: Ultimate analyses and ash contents of samples used
from a beverage company in Japan. This inherent water
retained by the coffee grounds was used for the hot water
Sample Ultimate analyses (wt%, d.a.f.)
treatment and for combining with calcium hydroxide to
ensure that Ca-loading was evenly dispersed. The coffee C H O+S N Ash
grounds were ground into particles less than 500 µm before (diff.) (wt%, d.b.)
use. A commercial microcrystalline cellulose (Nakalai Cellulose 44.4 6.2 49.4 N.D. 0.0
Tesque Inc.), organosolv-lignin (Sigma-Aldrich Co.), and Lignin 62.2 5.7 32.1 N.D. 3.4
xylan (Sigma-Aldrich Co.) were used to study the individual Xylan 42.7 5.8 51.5 N.D. 3.9
Coffee grounds 55.1 7.2 35.4 2.3 2.6
components of biomass. For the oil used in dewatering, any
Rapeseed oil 79.9 11.7 8.4 N.D. 0.0
oil that is immiscible with water can be used. In this work,Pretreatments for gasification of coffee grounds 171
gaseous products were collected using a gasbag and ana- content was measured with a Karl–Fisher moisture titrator
lyzed with a gas chromatograph. (Kyoto Electronics Manufacturing Co., Ltd., MKS-510N).
The dewatering in oil was conducted on the wet The analytical results, except for thermogravimetric ana-
coffee grounds as follows. The biomass sample was mixed lysis and X-ray diffraction, for the obtained samples are
with rapeseed oil in the ratio of 1 to 10 by weight in a hard expressed as the average of the three times.
glass tube reactor. It was then heated to a temperature
between 25°C and 230°C at atmospheric pressure by
immersing the reactor in a temperature-regulated oil
bath. Through this treatment, the coffee grounds became 3 Results and discussion
swollen with some oil and the moisture was removed
from the samples to a certain degree. Separation of the
3.1 Changes in the coffee grounds’
grounds from the oil adhered to its exterior was con-
ducted by filtration. As combined pretreatments, the properties through hot water treatment
dewatering at 150°C was also performed on the coffee
grounds mixed with Ca(OH)2 followed by the hot water Carbonization is one method to convert low-grade bio-
treatment at 230°C. mass into a calorific solid fuel, but the yield of char is
usually very low. Therefore, the effects of the hot water
treatment on the elemental composition of the coffee
grounds were examined to improve their calorific value.
2.4 Gasification Figure 1 shows the changes in the oxygen/carbon (O/C)
atomic ratio of the coffee grounds after the hot water
The reactivities of the as-received and the treated coffee treatment without extra water. The O/C atomic ratio
grounds were isothermally measured using a thermo- decreased with an increase in the temperature of the
gravimetric analyzer (Shimadzu Co., TGA-50). Approximately hot water treatment. The O/C ratio reached the minimum
2 mg of the coffee grounds was mounted on a platinum value of 0.26 at 230°C. Considering that the coffee
cell and heated at a rate of 20 K min−1 up to 900°C under a grounds were not carbonized to a significant extent under
flow of pure nitrogen gas and maintained at 900°C for a nitrogen atmosphere at 230°C, the water retained by the
30 min. Then, the nitrogen gas was replaced by CO2 gas at coffee grounds must have played a role in the decompo-
a constant temperature (600–900°C) to gasify the char sition of the functional groups containing oxygen. Given
with CO2. The char conversion is expressed as a weight that the higher heating value of the grounds treated at
percent on a dry, ash-free basis. 230°C corresponds to 29.1 MJ/dry-kg, calculated using
the Dulong’s formula, the hot water treatment is effective
at raising the calorific value of low-grade food waste
(22.6 MJ/dry-kg) using hygroscopic moisture. Therefore,
2.5 Analyses
0.6
Ultimate analyses of the samples were performed using
an elemental analyzer (BEL Japan, Inc., ECS4010). Any 0.5
solid chemical structures, such as functional groups,
were analyzed using an FTIR spectrometer (JEOL. Ltd., 0.4
JIR-SPX60). The TOC and the concentration of saccha-
O/C [-]
rides in the aqueous solution were estimated using a 0.3
TOC analyzer (Shimadzu Co., TOC-VCHS) and an LC
0.2
(Shimadzu Co.), respectively. For the LC analysis, an aqu-
eous mixture containing 70% acetonitrile was used as 0.1
the mobile phase and was fed at 1 mL min−1 to the LC
equipped with a column (TOSOH Co., TSKgel Amide-80) 0.0
untreated hot water hot water dewatering
and a refractive index detector. The crystallinity of the
180 °C 230 °C 150 °C oil
cellulose and the form of the calcium compounds were
determined using X-ray diffractometry (Shimadzu Co., Figure 1: Changes in the O/C atomic ratio of the coffee grounds after
XD-610, Cu-Kα, λ = 1.54 Å) at 30 kV and 30 mA. The water the hot water treatment or dewatering.172 Isao Hasegawa et al.
the treated coffee grounds could be successfully used in 100
combustion if they were sufficiently dewatered. In addi- gas
water-soluble
Carbon conversion [%]
tion, the gasification of biomass is a promising method 80
to produce syngas because of its high reactivity at low
temperatures. From this viewpoint, the effects of the 60
hot water treatment on the gasification reactivity of the
coffee grounds’ char were examined. Figure 2 shows 40
the CO2 gasification profiles at 900°C for the untreated
and treated coffee grounds’ chars. The gasification reac- 20
residue
tivity dramatically decreased with an increase in the tem-
perature of the hot water treatment. It was also observed
0
that the gasification reaction went to completion only 25 °C 180 °C 180 °C 230 °C 230 °C
after 8,800 s in the case of coffee grounds treated at extra H2O extra H2O
230°C, which was 20 times longer than for untreated
coffee grounds. From this result, it was determined that Figure 3: Carbon distributions under the several conditions of the
hot water treatment with/without extra water addition.
the coffee grounds treated in hot water were unsuitable
for gasification, likely because they were deactivated.
Next, the rationale for the deactivation of the coffee resulted in a higher conversion to gas than that of woody
grounds through the hot water treatment was investi- biomass in our previous study [19]. This is thought to be
gated. Feng et al. [18] reported that the activation energy because of the decomposition of water-soluble organic
was reduced in the gasification of sewage sludge char compounds containing functional groups produced by
after a hydrothermal treatment. Figure 3 compares the oxidation during the roasting of coffee beans. In our pre-
carbon distributions of the various hot water treatment vious study, it was determined that the hemicellulose
conditions with and without the addition of extra water. fraction in biomass could be recovered as saccharides
The gaseous products mainly consisted of CO2 gas. With through a hot water treatment. The coffee grounds con-
an increase in the hot water temperature, the yield of the tain hemicellulose, cellulose, lignin, proteins, and other
treated solid residue decreased. In contrast, the yield extracts, such as caffeine. The yields of the saccharide,
of water-soluble organic compounds showed almost no one of the water-soluble compounds, at each condi-
change at temperatures above 180°C. This indicates that tion were found to be 2.9 kg/100 kg-dry coffee grounds
the coffee grounds contained a fixed quantity of a mate- at 230°C, 3.1 kg/100 kg at 230°C with extra water,
rial that dissolves in hot water. At 230°C, a portion of 2.7 kg/100 kg at 180°C, 3.0 kg/100 kg at 180°C with extra
these water-soluble compounds appear to have been water, and 0.07 kg/100 kg at 25°C with extra water. With
decomposed into CO2 gas. Hot water treatment at 230°C the addition of extra water, the yields of the saccharide
increased slightly. It is likely that the hydrolysis of hemi-
cellulose was promoted in the presence of a large amount
1.0 of water because of autohydrolysis by the organic acid
CO2 products. Therefore, it may be possible to control the
hot water 230 °C
0.8 900 °C hydrolysis product distribution by regulating the
amount of moisture or extra added water. From the per-
spectives of saving energy and reducing the wastewater
0.6
1-X [-]
discharged from the treatment, no extra water was
hot water 180 °C added in the later hot water treatments. Given that
0.4
Minowa et al. [20] reported that cellulose is also hydro-
untreated lyzed at temperatures above 250°C, the hot water treat-
0.2
ment at 230°C is a method to produce solid residue in
good yields under mild conditions.
0.0 The hemicellulose in the coffee grounds was hydro-
0 2000 4000 6000 8000
lyzed into a certain amount of saccharide through the hot
Time [sec]
water treatment mentioned above. One reason that the
Figure 2: CO2 gasification profiles at 900°C for the untreated and treated coffee grounds were deactivated and unsuitable
treated coffee grounds’ chars. for gasification is that hemicellulose was released fromPretreatments for gasification of coffee grounds 173
the biomass, leaving behind lignin or other humid com- When wet biomass is dewatered in oil, the moisture is
pounds that are difficult to decompose. Other structural replaced with the oil. The success of this method relies
changes in the solid residue were also investigated. on the affinity of the biomass for oil and water. Thus, in
Figure 4 shows the FTIR spectra of the untreated coffee this section, the dewatering was performed using vege-
grounds and those treated with hot water. The spectrum table oil and any swelling of the biomass with oil was
of the coffee grounds treated at 230°C was strikingly dif- examined. If waste oil is used, the final biomass and
ferent from the untreated coffee grounds. The hot water oil mixture would be an excellent fuel for combustion
treatment at 230°C caused a decrease in the amount of because of its high-calorific value. First, to study each
hydroxyl groups (assigned at 2,400–3,700 cm−1). While component of the biomass, including hemicellulose, cel-
the hydroxyl groups decreased, the intensity of the lulose, and lignin, they were impregnated with oil at
peaks corresponding to carbonyl groups (assigned at room temperature and the oil uptake was measured.
1,630–1,780 cm−1) increased, indicating that the hot water Figure 5 shows the amount of oil uptake for the single-
treatment formed a cross-linked structure from the dehy- component samples and for cellulose treated in hot water
dration reaction of the functional groups. This cross-linked at 250°C. Lignin and the hydrothermally treated cellulose
structure would lead to deactivation in the gasification were swollen with a large amount of the oil, ca. 60 wt%.
reaction for the treated coffee grounds’ char. Therefore, In contrast, the measured oil content of both xylan
the utilization of hydrothermally treated coffee grounds (representative of hemicellulose) and cellulose was
as a feedstock for gasification requires a suppression of smaller than 20 wt%. There are two possible explanations
the formation of cross-links. A hot water treatment where for these observations. The first is attributed to the sur-
the coffee grounds can be upgraded without deactivation face properties of biomass. Biomass is classified as a
is described in Section 3.3. hydrophilic material because it contains a large quantity
of functional groups, such as hydroxyl groups. Looking
at each component of biomass, polysaccharides, such
as hemicellulose and cellulose, are richer in functional
3.2 Dewatering in oil groups compared to lignin. As described in Section 3.1,
when pure cellulose undergoes a hot water treatment, the
In cases where a waste or virgin wet biomass feedstock is O/C atomic ratio value drops from 0.8 to 0.4, where the
thermally processed for energy recovery, it may be neces- latter value matches lignin’s O/C ratio. In other words,
sary to partially dry or dewater the raw feed before the the more hydrophobic surfaces the biomass contains, the
subsequent conversion. Solar drying in open air is a low- more oil it can absorb. The second explanation for the
cost method for moisture reduction, but most food waste high oil absorption by the hydrothermally treated cellu-
or biomass with high-water content will decompose or lose is the collapse of the crystalline structure of cellulose
decay under these conditions. A spontaneous ignition is through the hot water treatment. The crystallinity of the
even possible because of the self-heating of oxidation. cellulose decreased with higher temperatures of the hot
80
O-H Ca(OH)2 C=O
hot water
Absorbance [a.u.]
230 °C 60
untreated
oil uptake [%]
40
hot water
230 °C
20
0
4000 3500 3000 2500 2000 1500 1000 lignin xylan cellulose cellulose
-1
Wavenumber [cm ] hot water 250 °C
Figure 4: FTIR spectra of the coffee grounds treated in hot water and Figure 5: Oil uptakes for the pure biomass components and the
of the untreated ones. cellulose treated in hot water.174 Isao Hasegawa et al.
water treatment, as shown in Figure 6. By relaxing the 60 60
firm crystalline structure, which previously prevented the
50 50
oil from penetrating into the hydrogen bonding formed
water content [%]
by the hydroxyl groups, the biomass can become highly
oil uptake [%]
40 40
swollen with oil. Therefore, a modification of the che-
mical or physical structure of the biomass feedstock 30 30
was found to be an effective method to adjust the oil
uptake. 20 20
Next, for the dewatering of wet biomass, the effect of
10 10
the water content on the amount of oil uptake was exa-
mined. The change in the oil uptake of wet coffee grounds 0 0
as a function of oil temperature during dewatering is 0 50 100 150 200 250
oil temperature [°C]
shown in Figure 7. Figure 7 also shows the changes in
the water content of the wet coffee grounds as a function Figure 7: Changes in the oil uptake and water content for wet coffee
of the oil temperature. With an increase in the oil tem- grounds with the oil temperature.
perature, the oil uptake for the coffee grounds increased
and the water content decreased. In other words, for wet
biomass, the oil was absorbed simultaneously with the groups are altered from strong to weak hydrogen bond-
rapid removal of water from the pores at temperatures ing, the oil can more easily penetrate into the pores.
greater than 100°C. This phenomenon is likely because When heated to 100°C and above, the water adsorbed
of the suction pressure caused by the evaporation of on the surface evaporates. In addition, the oil fills the
moisture. By taking advantage of this effect, the wet bio- gaps that were previously occupied by the evaporated
mass was almost completely dewatered at 150°C and water, replacing the water as the adsorbate. As the oil
was impregnated with the same weight of oil as coffee penetrates between the functional groups, an inhibition
grounds. As shown in Figure 1, the O/C atomic ratio of the of spontaneous ignition and a water-repellent effect are
coffee grounds dewatered at 150°C was the same as predicted, in addition to an improvement in the calorific
untreated grounds. Unlike in the hot water treatment, value.
the elemental composition of the coffee grounds did not
change after oil dewatering. This is likely because the
coffee beans were already parched at around 200°C and 3.3 Addition of catalyst precursor for
the coffee grounds did not chemically react in the oil. gasification
To summarize the above discussion, the oil cannot
penetrate into the hydrophilic pores of wet biomass at Wet food biomass waste was treated with hot water to
room temperature. If the structures of the functional reduce the O/C atomic ratio and was dewatered in vegetable
oil as mentioned above. Furthermore, the hydrothermally
treated coffee grounds were significantly deactivated
4
against gasification. In an attempt to make the coffee
untreated
hot water 240 °C grounds more active for gasification, calcium com-
3 pounds, which are a known gasification catalyst [21–23],
Intensity [a.u.]
were added to the coffee grounds. First, the type of cal-
hot water 260 °C cium species for use as the catalyst was chosen. The
2 coffee grounds were impregnated with a saturated aqu-
eous solution of either Ca(OH)2 or Ca(CH3COO)2. The solu-
bility of Ca(CH3COO)2 in water (34.7 g/100 g of water at
1
20°C) is much higher than that of Ca(OH)2 (0.16 g/100 g
of water at 25°C). Consequently, this difference in solubi-
0 lity results in a disparity in the quantity of Ca loading
16 18 20 22 24 26 28 30
in the coffee grounds. To avoid this problem, the wet
2T [degree]
coffee grounds were also directly mixed with Ca(OH)2
Figure 6: Changes in the crystallinity of cellulose through the hot powder so that the amount of Ca-loading was the same
water treatment. as that prepared from the saturated Ca(CH3COO)2 solution.Pretreatments for gasification of coffee grounds 175
1.0 coals. In the present study, the coffee grounds directly
CO2 mixed with Ca(OH)2 powder demonstrated a high reactivity
900 °C
0.8 Ca(CH3COO)2 aq. by the similar physical blending. This may be because the
(39wt%) coffee grounds contain a large quantity of functional groups
0.6 untreated and water. The greatest catalytic effect of the coffee grounds
1-X [-]
soaked in saturated Ca(CH3COO)2 was determined to be
0.4 because of the dispersion and the amount of catalyst.
Ca(OH)2 aq.
(loading of 5wt% Ca)
Figure 9 shows the X-ray diffraction patterns of the chars
Ca(OH)2
0.2 powder of the Ca-loaded coffee grounds made from soaking in a Ca
(37wt%) (CH3COO)2 solution and from mixing with dry Ca(OH)2
powder. The Ca-loaded chars, both from Ca(CH3COO)2 solu-
0.0
0 100 200 300 400 tion and from Ca(OH)2 powder, gave the same XRD pattern
Time [sec] as pure CaO. This result suggests that the precursors of the
catalyst, Ca(CH3COO)2 and Ca(OH)2, were decomposed into
Figure 8: CO2 gasification profiles at 900°C for some kinds of
CaO during the pyrolysis of the coffee grounds, and there-
Ca-loaded grounds’ chars.
fore, it is CaO that resulted in catalysis of the char gasifica-
tion. Based on this study, direct mixing with Ca(OH)2
powder was chosen as an economical and simple method
Figure 8 shows the CO2 gasification profiles at 900°C for the of adding a Ca precursor to coffee ground waste.
Ca-loaded coffee grounds’ char. The coffee grounds soaked Finally, the above three treatments were combined.
with the saturated Ca(OH)2 solution showed almost the The coffee grounds mixed with Ca(OH)2 followed by the
same reactivity as untreated coffee grounds. This result is hot water treatment were investigated. Figure 10 shows
likely because of the negligibly small amount of Ca loading. the O/C atomic ratio of the coffee grounds after the treat-
In contrast, the coffee grounds soaked with the saturated Ca ment combination. Table 2 shows the elemental compo-
(CH3COO)2 solution and the grounds directly mixed with Ca sitions of those. The coffee grounds with Ca(OH)2 added
(OH)2 powder demonstrated a significant catalytic effect. followed by the hot water treatment did not demonstrate
From these results, it was determined that the gasification a significant reduction in the O/C ratio in comparison to
reactivity of the coffee grounds depends strongly on the those treated using hot water only. This result suggests
amount of Ca loading in the coffee grounds. It was reported that Ca(OH)2 acts as an inhibitor against the dehydration
that bituminous coal physically mixed with CaO did not reaction between the functional groups. The resulting
show much catalytic effect in gasification [24]. The authors suppression of the cross-linked structure was also con-
concluded that it was because of the lack of carboxyl firmed, as shown in Figure 4. With the addition of Ca
groups. Ohtsuka and Asami [21] reported that Ca(OH)2 at (OH)2, the hydroxyl groups that form the stronger
a loading of 5 wt% Ca promotes the steam gasification of hydrogen bonds (assigned at 2,600–3,300 cm−1) did not
0.6
0.5
Ca(CH3COO)2 aq. char
Intensity [a.u.]
0.4
O/C [-]
0.3
Ca(OH)2 powder char
0.2
pure CaO 0.1
0.0
untreated hot water Ca(OH)2 addition
10 20 30 40 50 60 230 °C hot water
hot water &
2T [degree] 230 °C dewatering
Figure 9: X-ray diffraction patterns for the chars of the Ca-loaded Figure 10: Changes in the O/C atomic ratio of the coffee grounds
coffee grounds and CaO. after the combined treatments.176 Isao Hasegawa et al.
Table 2: Ultimate analyses of the treated coffee grounds and their 1.0
char yields at 900°C
hot water 230 °C CO2
0.8 900 °C
Treatments Ultimate analyses (wt%, d.a.f.)
untreated
C H O+ N 900°C 0.6
1-X [-]
Ca(OH)2
S (diff.) char yield
hot water
Hot water at 230°C 67.0 7.4 23.2 2.4 0.28 0.4 & dewatering
Ca-loading and HW 59.9 6.9 30.7 2.5 0.20
Ca(OH)2
at 230°C
0.2
Ca-loading and HW at 58.4 6.8 32.3 2.5 0.15 Ca(OH)2
230°C followed by hot water
dewatering at 150°C 0.0
0 50 100 150 200
Time [sec]
significantly decrease through the hot water treatment. Figure 11: CO2 gasification profiles at 900°C for the Ca-loaded coffee
Figure 11 compares the gasification rates of the chars of grounds’ chars after the combined treatments.
the coffee grounds treated with the various methods. As
compared with the untreated coffee grounds, the coffee
produced an inactive cross-linked structure attributed
grounds treated only with hot water at 230°C demon-
to dehydration. During the dewatering process in vege-
strated a considerable decline in the gasification rate.
table oil at 150°C, water was completely removed from
In contrast, all coffee grounds samples mixed with Ca
the coffee grounds and the grounds were impregnated
(OH)2 showed a drastic increase in the gasification rate.
with a large amount of oil. By physically mixing the coffee
Dewatering in oil did not affect the gasification rate of the
grounds with Ca(OH)2 powder in advance of the hot water
chars. These results confirm that Ca(OH)2 suppressed
treatment, cross-linking was suppressed and gasification
the cross-linking in the hot water treatment and acted
rate increased significantly because of the catalytic effect
as a catalyst for gasification. Finally, the catalytic effect
of Ca. With or without hot water treatment, the time
was quantitatively analyzed using reaction kinetics. From
required to complete gasification at 900°C for the chars
the Arrhenius plots of the gasification rates at conver-
of the coffee grounds mixed with calcium hydroxide was
sions of 0.5 for the coffee grounds with and without Ca
reduced to about one-third of that for the char of the
(OH)2, the calculated apparent activation energies were
untreated coffee grounds. In summary, the combined
as follows: 286 kJ mol−1 for the untreated coffee grounds
method described herein is an effective approach to
and 239 kJ mol−1 for the Ca-catalyzed coffee grounds.
upgrade wet biomass into a valuable feedstock for con-
Overall, the Ca-loaded coffee grounds were certainly
ventional thermochemical conversion processes.
reactive, and an increase in the gasification rate was
demonstrated using catalyzed coffee grounds as a gasifi-
Research funding: This work was financially supported
cation feedstock.
by the NEDO (New Energy and Industrial Technology
Development Organization) “Development of Efficient
Conversion Technology for Biomass Energy.”
4 Conclusions Author contributions: Isao Hasegawa: writing – original
draft, review and editing, methodology, validation,
A new pretreatment method for efficiently using wet formal analysis, and visualization; Tatsuya Tsujiuchi:
biomass as a high calorific and reactive feedstock for writing – review and editing, investigation, formal ana-
gasification was presented. The method consists of three lysis, visualization, and data curation; Kazuhiro Mae:
treatments: addition of Ca(OH)2, hot water treatment, and writing – review and editing, resources, conceptualiza-
dewatering in oil. The impacts of the operating condi- tion, project administration, funding acquisition, and
tions of these treatments on the properties of the treated validation.
biomass were examined, and the following conclusions
were obtained. Hot water treatment at 230°C reduced Conflict of interest: The authors state no conflict of
the O/C atomic ratio of the coffee grounds to 0.26 and interest.Pretreatments for gasification of coffee grounds 177
Data availability statement: All data generated or ana- [11] Lang RJ, Neavel RC. Behaviour of calcium as a steam gasifi-
lyzed during this study are included in this published cation catalyst. Fuel. 1982;61:620–6.
article. [12] Leppalahti J, Simell P, Kurkela E. Catalytic conversion of
nitrogen-compounds in gasification gas. Fuel Process Technol.
1991;29:43–56.
[13] Zheng ZM, Luo LX, Feng AW, Iqbal T, Li ZY, Qin W, et al. CaO-
assisted alkaline liquid waste drives corn stalk chemical
looping gasification for hydrogen production. ACS Omega.
References 2020;38:24403–11.
[14] Sarafraz MM, Jafarian M, Arjomandi M, Nathan GJ. Potential of
[1] Moreira RG. Impingement drying of foods using hot air and molten lead oxide for liquid chemical looping gasification
superheated steam. J Food Eng. 2001;49:291–5. (LCLG): a thermochemical analysis. Int J Hydrog Energ.
[2] Iyota H, Konishi Y, Yoshida K, Nishimura N, Nomura T, 2018;43:4195–210.
Yoshida M. Drying of carbohydrate food in superheated steam [15] Hasegawa I, Tabata K, Okuma O, Mae K. New pretreatment
and hot air – characteristics of coloring of potato slice sur- methods combining a hot water treatment and water/acetone
faces. Kagaku Kogaku Ronbunshu. 2003;29:94–9. extraction for thermo-chemical conversion of biomass. Energy
[3] Miura K, Mae K, Ashida R, Tamura T, Ihara T. Dewatering of coal Fuels. 2004;18:755–60.
through solvent extraction. Fuel. 2002;81:1417–22. [16] Fujitsuka H, Ashida R, Miura K. Upgrading and dewatering of
[4] Adschiri T, Hirose S, Malaluan R, Arai K. Noncatalytic conver- low rank coals through solvent treatment at around 350
sion of cellulose in supercritical and subcritical water. J Chem degrees C and low temperature oxygen reactivity of the treated
Eng Jpn. 1993;26:676–80. coals. Fuel. 2013;114:16–20.
[5] Xu XD, Matsumura Y, Stenberg J, Antal MJ. Carbon-catalyzed [17] Sakaguchi M, Laursen K, Nakagawa H, Miura K. Hydrothermal
gasification of organic feedstocks in supercritical water. Ind upgrading of Loy Yang Brown coal – effect of upgrading con-
Eng Chem Res. 1996;35:2522–30. ditions on the characteristics of the products. Fuel Proc
[6] Elliot DC, Neuenschwander GG, Phelps MR, Hart TR, Zacher AH, Technol. 2008;89:391–6.
Silva LJ. Chemical processing in high-pressure aqueous [18] Feng YH, Yu TC, Chen DZ, Xu GL, Wan L, Zhang Q, et al. Effect of
environments. 6. Demonstration of catalytic gasification hydrothermal treatment on the steam gasification behavior of
for chemical manufacturing wastewater cleanup sewage sludge: reactivity and nitrogen emission. Energy Fuels.
in industrial plants. Ind Eng Chem Res. 2018;32:581–7.
1999;38:879–83. [19] Mae K, Hasegawa I, Sakai N, Miura K. A new conversion
[7] Antal MJ, Allen SG, Schulman D, Xu XD, Divilio RJ. Biomass method for recovering valuable chemicals from oil palm shell
gasification in supercritical water. Ind Eng Chem Res. wastes utilizing liquid-phase oxidation with H2O2 under mild
2000;39:4040–53. condition. Energy Fuels. 2000;14:1212–8.
[8] Matsumura Y, Minowa T, Potic B, Kersten SRA, Prins W, van [20] Minowa T, Zhen F, Ogi T. Cellulose decomposition in hot-
Swaaij WPM, et al. Biomass gasification in near- and super- compressed water with alkali or nickel catalyst. J Supercrit
critical water: Status and prospects. Biomass Bioenerg. Fluids. 1998;13:253–9.
2005;29:269–92. [21] Ohtsuka Y, Asami K. Steam gasification of coals with calcium
[9] Yan M, Su HC, Zhou ZH, Hantoko D, Liu JY, Wang JY, et al. hydroxide. Energy Fuels. 1995;9:1038–42.
Gasification of effluent from food waste treatment process in [22] Wang J, Morishita K, Takarada T. High-temperature inter-
sub- and supercritical water: H2-rich syngas production and actions between coal char and mixtures of calcium oxide,
pollutants management. Sci Total Environ. 2020;730:138517. quartz, and kaolinite. Energy Fuels. 2001;15:1145–52.
doi: 10.1016/j.scitotenv.2020.138517. [23] Dalai AK, Sasaoka E, Hikita H, Ferdous D. Catalytic gasification
[10] Su W, Cai CQ, Liu P, Lin W, Liang BR, Zhang H, et al. of sawdust derived from various biomass. Energy Fuels.
Supercritical water gasification of food waste. Effect of 2003;17:1456–63.
parameters on hydrogen production. Int J Hydrog Energy. [24] Johnson JL. The use of catalysts in coal gasification. Catal Rev
2020;45:14744–55. Sci Eng. 1976;14:131–52.You can also read
